1. Field of the Invention
This invention relates to high precision instruments known as dilatometers, which are designed to measure dimensional changes of a specimen brought about by changes in its environment; and more particularly, to such dilatometers configured for ratiometric measurement between two or more parallel plate capacitors, and related methods, such instruments being referred to herein as “ratiometric capacitance dilatometers”.
2. Description of the Related Art
The coefficient of thermal expansion is a fundamental property of all materials; yet the capability of measuring thermal expansion is not readily available in most laboratories, especially at cryogenic temperatures on the order of few or a fraction of a degree Kelvin.
A dilatometer is an ultra-sensitive instrument for measuring dimensional changes of a material brought about by changes in its environment. Various applications for dilatometers may include: locating phase transitions in materials; predicting pressure effects in superconductors; characterizing cryogenic construction materials; magnetostriction studies or providing information that is complementary to heat capacity data.
A number of dilatometers have been proposed in the art, including: furnace and push rod dilatometers used in high temperature applications; resonant frequency dilatometers used in low temperature applications; piezo-resistive dilatometers which have been shown to provide modest resolution; and capacitive dilatometers which are the most sensitive of these instruments but also the most difficult to successfully implement. The embodiments herein will relate to improved capacitive dilatometers.
The capacitance dilatometer is designed to benefit from the ability to accurately measure changes in capacitance between two parallel plates. Because of this ability, such dilatometers are capable of measuring length changes on the order of a fraction of an Angstrom.
There is an interest in studying thermal expansion of various solids, and in particular, such solids under very low temperatures and/or within an applied magnetic field.
Thus, in addition to measuring thermal expansion of a material, it would be beneficial to collect data relating to dimensional changes of a given sample at very low temperatures and within various magnetic fields or gradients. Accordingly, various embodiments herein have been designed for use with the commonly owned and commercially available “Physical Property Measurement System” or “PPMS” of Quantum Design, Inc., which is a versatile, low temperature cryostat capable of providing an environment with temperatures between 0.05 K and 1000K, and with magnetic fields up to 16 T. The PPMS further provides automated, on-board temperature controlling and measuring ability. Although the PPMS is an exceptional platform for use with certain embodiments herein, it should be understood that other low temperature cryostats or similar systems may be similarly implemented and that the scope of the invention is not intended to be limited to practice with the above-described PPMS.
Recent advancements in the design and implementation of capacitive dilatometers are described in Schmiedeshoff et al., “Versatile and compact capacitive dilatometer”, Review of Scientific Instruments 77, 123907 (2006). Schmiedeshoff describes a capacitive dilatometer having cylindrical geometry and fabricated from Copper. Copper is purportedly selected for its high thermal conductivity, machinability, relative insensitivity to high magnetic fields, and well known thermal expansion characteristics. However, although well known, copper alloys have high thermal expansions, and suffer from magnetic torque on induced eddy currents which result in a large contribution to the raw expansion data from the cell itself. This thermal expansion portion of the data must be subtracted in order to determine the expansion component attributed to the material sample. This correction is widely referred to as the “empty cell effect”, whereas the correction takes into consideration an amount of noise attributed to thermal expansion of the empty cell.
Furthermore, miniature capacitance-based dilatometer expansion cells are almost universally constructed out of copper, copper beryllium or other copper alloys. This construction however suffers from the fact that insulating materials are needed to electrically isolate the capacitance plates from the body of the expansion cell which is also conducting. In practice, these copper alloy expansion cells will typically result in a large “empty cell effect” or background signal due to the large thermal expansion of the conductive materials and or the complex copper and insulating construction of the cell itself.
More recently, Neumeier et al. describe a dilatometer cell that can detect sub angstrom changes in length of solid specimens within the temperature range 5 K<T<350 K in “Capacitive-based dilatometer cell constructed of fused quartz for measuring the thermal expansion of solids”, Review of Scientific Instruments 79, 033903 (2008). The Neumeier dilatometer is fabricated from an insulating fused silica (quartz), which provides low thermal expansion, and thus exhibits a smaller contribution of the cell's thermal expansion to the raw data for a reduced “empty cell effect”.
FIG. 1 illustrates the Neumeier cell having a stationary L-shaped base piece with a capacitor plate formed on an inner vertical surface, a moveable L-shaped piece having a second capacitor plate formed on a vertical surface configured to oppose the first capacitor plate, a wedge for wedging a sample between the L-shaped pieces, and a pair of springs for maintaining a counterpoise force against the direction of sample expansion. The cell body is composed of fused silica, or quartz.
Benefits of the Neumeier dilatometer cell include: low expansion due to the all fused-silica construction; the cell is not sensitive to magnetic field; the cell (per se) is not sensitive to temperature; the capacitance readout gives high resolution; large size gives a sizeable capacitance to measure; an Andeen-Hagerling bridge is an excellent off-the-shelf solution for capacitance readout; and the cell can accommodate a range of sample lengths.
Although an improvement over prior copper alloy type capacitive dilatometers, which themselves suffer from high thermal expansion, in practice, the Neumeier cell presents several problems when used within a low temperature cryostat, such as: capacity is affected by gas adsorption; an absolute capacitance measurement is required; and thermal gradients affect accuracy in 1st order.
Moreover, in all of these capacitive prior art cells, the measurement is affected by gas adsorption and thermal gradients to first order; and measurement of the absolute capacitance is required which is difficult without very specialized and expensive instrumentation.
With the intense interest in expansion measurements of material solids, there continues a need for an improved capacitive dilatometer cell which addresses these and other practical needs in the art.